Narrator: CALL IT..."ANIMAL ATTRACTION."

THE MATING GAME.

COURTSHIP DRIVES EVOLUTION BY CONTROLLING WHOSE GENES ARE PASSED ON TO THE NEXT GENERATION.

COMPETITION GIVES RISE TO DAZZLING DISPLAYS AND IMPRESSIVE ORNAMENTS.

CHARLES DARWIN CALLED IT "SEXUAL SELECTION."

NOW, SCIENTISTS ARE UNLOCKING ONE OF ITS MOST SURPRISING SECRETS -- THE IMPORTANCE OF FEMALE CHOICE.

Woman: THE ROBOT CAN CROUCH UP AND DOWN.

Narrator: USING NEW TECHNOLOGY, THEY'RE FINDING OUT WHAT FEMALES LOOK FOR IN A MATE.

Woman: ALL RIGHT, EVERYONE, HERE COMES THE ROBOT.

Narrator: A MALE'S COLORS HELP HIM SHOW WHAT HE'S MADE OF.

MAKES YOU WONDER WHAT ARE THESE GIRLS CHOOSING?

WOULD YOU GO FOR HIS NICE PINK CHEST PATCH?

Narrator: BUT RIVALRIES CAN TURN UGLY.

THERE'S SO MUCH TENSION HERE THIS MORNING.

[ SHRIEKING ] Narrator: THE RIGHT LOOK CAN CHARM THE LADIES, THOUGH IT MAY BE JUST A DISGUISE.

THIS LUCKY GUY IS GOING TO GET A SERIOUS MAKEOVER.

SHE LIKES HER COLORS, SHE LIKES HER SOUNDS.

SHE'S ENJOYING BEING ALIVE, AND SHE WANTS A SHOW.

Narrator: AND EVEN AFTER THEY FIND A MATE, FEMALES KEEP LOOKING.

THERE IS A LOT OF CHEATING GOING ON, AND MORE IS MORE.

Narrator: THIS NEW VIEW OF FEMALE CHOICE IS OPENING OUR EYES TO THE MYSTERIOUS ALCHEMY OF ATTRACTION.

Man: NEVER SEEN THIS BEFORE.

Narrator: TO GET A NEW PERSPECTIVE ON THE MATING GAME, SCIENTISTS ARE HEADING TO REMOTE PLACES, LIKE THE NORTHER HIGHLANDS OF ETHIOPIA.

Man: HEY, GUYS -- HEY, CHILL, CHILL, IT'S JUST ME.

IT'S JUST ME.

Abraham: CHADDEN HUNTER IS TRACKING A TROOP OF GELADAS -- THEY'RE CLOSE COUSINS OF BABOONS.

[ YAWNING ] [ GELADAS CHATTERING ] AFTER SPENDING NEARLY A DECADE OBSERVING THIS TROOP, CHADDEN HAS GAINED THEIR TRUST... AND HE CAN MOVE AMONG THEM AT WILL.

THIS IS MORNING CHILL-OUT TIME.

MIGHT LOOK JUST LIKE THIS MASS OF MONKEYS, BUT IN HERE WE HAVE INDIVIDUAL FAMILY GROUPS.

AND THESE FEMALES PICK AND CHOOSE A MALE.

Abraham: MALE GELADAS ARE MUCH BIGGER THAN FEMALES.

THEY SPORT HANDSOME GOLDEN MANES AND A FEARSOME DEMEANOR.

BUT IN GELADA SOCIETY, THE FEMALES MAKE ALL THE DECISIONS...

INCLUDING THE CHOICE OF A MATE.

CHADDEN WANTS TO FIND OUT WHAT MAKES A MALE GELADA A GOOD CATCH.

WHAT DO THESE FEMALES WANT?

Hunter: I'M TRYING TO GET IN CLOSE TO CHEWBACCA HERE, WHO'S THE BIG MALE THAT I'VE BEEN TRYING TO FIND.

Abraham: CHEWBACCA IS A FAMILY MALE.

THIS LUCKY GUY ENJOYS THE COMPANY OF A HAREM OF SEVEN FEMALES.

ONLY FAMILY MALES GET TO MATE.

BUT THEY ONLY GET THE JOB IF THE FEMALES SELECT THEM.

Hunter: MAKES YOU WONDER, WHAT ARE THESE GIRLS CHOOSING?

IF YOU WERE A GELADA FEMALE, WOULD YOU GO FOR CHEWBACCA'S GOLDEN MANE?

WOULD YOU GO FOR HIS NICE PINK CHEST PATCH?

WOULD IT BE HOW ATTENTIVE HE IS?

HOW MUCH HE GROOMS YOU?

HOW MUCH HE LOOKS AFTER THE KIDS?

THAT'S WHAT I'M TRYING TO GET TO THE BOTTOM OF -- EXACTLY WHAT THESE GIRLS LOOK FOR WHEN THEY CHOOSE A MALE LIKE CHEWBACCA.

Abraham: CHADDEN IS ESPECIALLY INTERESTED IN THE MALE'S DISTINCTIVE CHEST PATCH -- A REDDISH TRIANGLE OF BARE SKIN.

ALL GELADAS HAVE CHEST PATCHES.

IN FEMALES, THEY ARE INDICATORS OF FERTILITY, LIKE THE COLORFUL SWELLINGS ON THE BUTTOCKS OF BABOONS.

Hunter: BECAUSE A GELADA SITS ON ITS BUTT ALL DAY PICKING GRASS, IT CAN'T SHOW ITS SEXUAL STATE.

SO, IT'S MOVED TO THE CHEST FOR THE GIRLS.

Abraham: WHEN A FEMALE GELADA IS READY TO MATE, HER CHEST PATCH LETS EVERYONE KNOW IT.

Hunter: SEE THAT FEMALE'S CHEST?

SHE'S GOT THESE BUBBLES AROUND THE OUTSIDE OF IT.

THAT'S TO SHOW SHE'S IN ESTROUS, TO SHOW THAT SHE'S FERTILE.

THAT'S WHY THE FEMALES HAVE A BARE CHEST.

WHAT I CAN'T UNDERSTAND IS WHY THE MALES HAVE THAT SAME BARE CHEST.

HEY, GUYS.

Abraham: THERE MAY BE A CLUE IN THE WAY THE MALE'S CHEST PATCH CHANGES COLOR.

IT CAN RANGE FROM A PALE PINK TO A BRIGHT RED.

CHADDEN THINKS THAT COLOR IS CONVEYING A MESSAGE TO FEMALES, TELLING THEM THAT A MALE IS HEALTHY, OR THAT HE HAS GOOD GENES.

Hunter: WHAT I WANT TO DO IS TRY AND GET CLEAR PICTURES OF THESE MALES SO I CAN COMPARE THE DIFFERENT QUALITIES OF THAT CHEST PATCH, AND SEE IF I CAN GET TO THE BOTTOM OF WHETHER THE FEMALES ARE USING THAT TO CHOOSE THEIR MALES.

Abraham: THE FIRST STEP IS TO CREATE A DIGITAL CATALOG OF THE MALES' CHEST PATCH COLORS... PHOTOGRAPHING THEM NEXT TO A STANDARD COLOR CHART FOR A CONTROLLED COMPARISON.

Hunter: PAPARAZZI TIME... [ SHUTTER CLICKING ] NICE CHEST.

OH, LOVELY.

OH, OUT OF THE WAY.

Abraham: CHADDEN IS AMASSING A LARGE COLLECTION OF GELADA PORTRAITS... OH, SWEET!

WITH CHEST PATCHES IN A RANGE OF SHADES.

Hunter: OH, GOOD, GOOD BOY.

Abraham: THIS LITTLE GUY'S TOO YOUNG TO MAKE THE CUT.

Hunter: WONDER HOW YOU'RE GONNA IMPRESS THE GIRLS, HUH?

HOW ARE YOU GONNA GET SELECTED?

Abraham: THERE'S NO DOUBT THAT CHEWBACCA IS ONE MALE WHO CAN IMPRESS THE GIRLS.

BUT EVEN WITH HIS HAREM OF FEMALES, HIS STATUS IS ALWAYS AT RISK.

Hunter: YOU CAN SEE THE MAIN HERD BACK HERE, WITH ALL THE FAMILIES.

NOW, THE GIRLS BACK HERE HAVE TO CHOOSE MALES.

AND THIS IS THE ELIGIBLE BACHELORS TO CHOOSE FROM, THE BACHELOR GANG.

ALL THESE GUYS ARE SINGLE.

THEY HANG OUT CLOSE TOGETHER.

Abraham: THE BACHELORS ROAM THROUGH THE HERD, EACH ONE HOPING TO CONVINCE FEMALES HE'S A BETTER CHOICE THAN THEIR CURRENT MATE.

Hunter: THIS GUY ON THE END HERE IS ELVIS.

HE'S THE BIGGEST AND BADDEST BACHELOR, AND HE'S CHEWBACCA'S BIGGEST CHALLENGE.

HE'S ABOUT AS ELIGIBLE AS A BACHELOR GETS.

Abraham: ELVIS FREQUENTLY TESTS CHEWBACCA, CHALLENGING HIM WITH A MENACING GLARE.

AND CHEWBACCA KNOWS HE CAN'T BACK DOWN.

ELVIS DECIDES NOT TO PUSH HIS LUCK...

THIS TIME.

FOR CHEWBACCA THERE WILL BE MORE CHALLENGES, AND THE NEXT CONFRONTATION COULD COST HIM HIS HAREM.

AT THE END OF THE DAY, CHADDEN CAN DOWNLOAD HIS PHOTOS TO A LAPTOP, TO BE SORTED BY COLOR... AND BY MATING SUCCESS.

HE KNOWS THESE MALES WELL, AND HE KNOWS HOW EACH ONE RATES WITH THE FEMALES.

Hunter: SEE THE LITTLE DOTS I'M PUTTING ON HIS CHEST?

AS I MOVE THE CIRCLE AROUND, ALL THE RATINGS DOWN THE SIDE CHANGE.

Abraham: IT'S CLEAR THAT FAMILY MALES LIKE CHEWBACCA AND PRIME BACHELORS LIKE ELVIS HAVE THE REDDEST CHEST PATCHES.

CHADDEN'S THEORY COULD BE CORRECT.

FEMALES MAY BE LOOKING FOR THE RIGHT COLOR WHEN THEY CHOOSE A MATE.

BUT PROVING IT WON'T BE EASY, AND IT COULD PUT CHADDEN IN THE MIDDLE OF A BATTLEGROUND.

[ BIRDS COOING ] THROUGHOUT THE ANIMAL KINGDOM, MALES DO THEIR BEST TO STAND OUT FROM THE CROWD AND MAKE SURE FEMALES NOTICE THEM.

THE COURTSHIP DISPLAY OF THE MALE SAGE GROUSE IS PRETTY HARD TO IGNORE.

WHEN HE WANTS TO ATTRACT A FEMALE'S ATTENTION, HE GIVES IT EVERYTHING HE'S GOT -- WITH COMPLEX CHOREOGRAPHY, AND A PECULIAR SONG HE PRODUCES BY INFLATING THE VOCAL SACS ON HIS CHEST.

[ THUMPING ] IF A FEMALE IS INTERESTED, SHE LETS HIM KNOW IT BY DRAWING CLOSER AND CROUCHING DOWN.

BUT THIS FEMALE IS A BIRD OF A DIFFERENT FEATHER.

Woman: THIS IS A FEMBOT.

THIS IS A ROBOTIC FEMALE SAGE GROUSE, AND THE MALES SEEM TO LIKE HER QUITE WELL.

Abraham: BIOLOGIST GAIL PATRICELLI IS TRYING TO FIGURE OUT EXACTLY WHAT FEMALES LOOK FOR IN A MATE... AND HOW THE MALES STRUT THEIR STUFF.

TO LEARN THEIR SECRETS, SHE'S DEVELOPED A ROBOT IN HER LAB AT THE UNIVERSITY OF CALIFORNIA.

THE MOST IMPORTANT THING THAT WE USE THE ROBOT FOR IS TO MEASURE THE DISPLAY OF THE MALE FROM THE PERSPECTIVE OF THE FEMALE.

AND WE DO THAT USING A MICROPHONE.

Abraham: GAIL BELIEVES FEMALE SAGE GROUSE ARE LESS INTERESTED IN A MALE'S MOVEMENTS THAN IN HIS SOUNDS.

THEY LISTEN TO GAUGE WHETHER HE'S WORTH CHOOSING.

THE MALES PERFORM THEIR RAUCOUS DISPLAYS IN EARLY SPRING, ON A LEK -- A STRETCH OF PRAIRIE RESERVED JUST FOR COURTING AND MATING.

IT'S SORT OF A SAGE GROUSE SINGLES BAR.

FIRST, MALES COMPETE FOR THE BEST TERRITORIES, WHERE THEY CAN SHOW THEMSELVES OFF TO GOOD ADVANTAGE.

THEN THE FEMALES CRUISE THE LEK, COMPARISON SHOPPING, TO CHOOSE THE MALE WHOSE DISPLAY THEY LIKE THE BEST.

FEMALES ARE VERY PARTICULAR.

BUT IN ANY LEK, A FEW MALES SEEM TO DO MOST OF THE MATING.

TO FIND OUT WHAT MAKES THOSE SUPER-BREEDERS SO SUCCESSFUL, GAIL WANTS TO STUDY THE FINER POINTS OF A MALE'S COURTSHIP DISPLAY, ESPECIALLY HIS VOCAL PERFORMANCE.

THAT'S WHERE THE FEMBOT COMES IN.

Patricelli: OKAY, SO, THE ROBOT HAS A FEW DIFFERENT WAYS THAT SHE CAN MOVE.

SHE CAN MOVE HER HEAD BACK AND FORTH, AND SHE CAN ALSO CROUCH UP AND DOWN.

AND SHE CAN ALSO TURN TO FACE THE MALE.

SHE ALSO HAS A VIDEO CAMERA ONBOARD, WITH A LITTLE WIRELESS TRANSMITTER.

Abraham: ON THIS CYBORG SAGE GROUSE, THE FEATHERS, AT LEAST, ARE GENUINE.

Patricelli: SO, TO DISGUISE THE ROBOTIC INNARDS, WE PUT THE REAL FEMALE SKIN ON TOP OF THE ROBOT.

Abraham: GAIL IS STRIVING FOR AUTHENTICITY, BUT HER TARGET AUDIENCE MIGHT PROVE TO BE A PUSHOVER.

Patricelli: THE MALES ARE NOT THAT PICKY ABOUT WHO THEY SPEND TIME COURTING.

WE OFTEN SEE MALES ON THE LEK TRYING TO MATE WITH COW PIES, SO THE BAR IS PRETTY LOW IN TRYING TO MAKE HER LOOK REAL, BUT WE WANT TO MAKE IT AS REALISTIC AS POSSIBLE SO THAT WE CAN ELICIT THE MOST NATURAL COURTSHIP BEHAVIORS POSSIBLE.

Abraham: TO PUT THE FEMBOT THROUGH HER PACES, GAIL'S TEAM WILL SET HER UP ON A REAL SAGE GROUSE LEK, ON THE WYOMING PRAIRIE.

Patricelli: AS STRAIGHT AS WE POSSIBLY CAN.

Abraham: THE FEMBOT WILL BE ROLLING ON A 30-FOOT TRACK.

BUT ROUGH TERRAIN AND HIGH WINDS COULD MAKE FOR A BUMPY RIDE.

Patricelli: THE ROBOT IS A BIT TOP-HEAVY... SHE'S RIDING ON THESE THIN RAILS, BUT SHE'S ABOUT THAT WIDE.

SO, WE'LL SEE IF SHE STAYS UPRIGHT.

Abraham: THE TRACKS WILL CARRY THE FEMBOT DEEP INTO THE HEART OF A MALE'S TERRITORY.

WHAT HAPPENS THEN IS ANYBODY'S GUESS.

BUT GAIL'S READY TO RECORD EVERY DETAIL.

TO FIND OUT JUST WHAT A FEMALE IS LISTENING TO, THE TEAM IS PUTTING TOGETHER AN ACOUSTIC MONITORING SYSTEM.

24 MICROPHONES WILL RECORD THE CALLS FROM EVERY ANGLE.

FINALLY, EVERYTHING'S READY.

NOW ALL THEY NEED ARE SAGE GROUSE.

FOR THAT, THEY'LL HAVE TO WAIT UNTIL TOMORROW.

NOT ALL BIRDS PUT ON A FLASHY DISPLAY LIKE SAGE GROUSE DO.

BARN SWALLOWS ARE SLEEK LITTLE SONGBIRDS THAT COME IN FAIRLY MUNDANE COLORS -- BLUES AND BROWNS, WITH CHESTNUT BROWN HIGHLIGHTS.

BUT TO A FEMALE BARN SWALLOW, THOSE COLORS MAY SPEAK VOLUMES ABOUT A MALE'S MERITS.

RESEARCHER REBECCA SAFRAN HAS DEVELOPED A THEORY THAT FEMALES ARE CHOOSING MALES WITH DARKER FEATHERS.

TO TEST THE IDEA, SHE'LL USE A MAGIC MARKER TO GIVE SOME MALES A MAKEOVER, AND MAYBE CHANGE THEIR LIVES.

FIRST SHE NEEDS TO ROUND UP A FEW VOLUNTEERS.

Safran: LET'S SEE WHO'S IN HERE.

[ CLAPPING ] COME ON, EVERYBODY OUT!

Abraham: REBECCA HAS ALREADY LEARNED THAT MALES WITH DARKER FEATHERS TEND TO BREED EARLIER AND FATHER MORE YOUNG.

SHE HOPES TO FIND OUT JUST WHY THAT'S TRUE.

Safran: ALL RIGHT, SUCCESS!

WE HAVE MALE BARN SWALLOWS.

Abraham: BEFORE THEY CAN ENTER THE PROGRAM, HER SUBJECTS HAVE TO UNDERGO A PHYSICAL AND DONATE A FEW FEATHERS, JUST FOR THE RECORD.

NOW IT'S TIME TO FIND OUT IF A NEW LOOK WILL BE A TURN-ON FOR THE FEMALES.

THIS LUCKY GUY IS GOING TO GET A SERIOUS MAKEOVER.

Abraham: SHE'S WORKING SOME MAGIC WITH A NON-TOXIC PERMANENT MARKER, THE SAME SHADE AS THE DARK BROWN FEATHERS ON THOSE MALES THAT GET THE GIRLS.

Safran: YOU CAN SEE THAT IT'S SIGNIFICANTLY DARKER THAN THE SIDE WHERE I DIDN'T MANIPULATE COLOR YET.

Abraham: IF THE FEMALES MATE WITH HER IMPOSTERS, IT COULD BE BECAUSE THEY VIEW DARK FEATHERS AS A SIGN OF GOOD GENES.

IT'S ANALOGOUS TO SUDDENLY OUTFITTING YOURSELF IN GUCCI AS OPPOSED TO LEVIS OR OLD NAVY.

SUPER STUD.

Abraham: A MAKEOVER COULD HELP THIS LITTLE GUY WIN A MATE, AND GET A NEW LEASE ON LIFE.

IT'LL BE A FEW WEEKS BEFORE REBECCA KNOWS FOR SURE.

BARN SWALLOWS, LIKE SWANS AND MORE THAN 90% OF ALL BIRD SPECIES, FORM LASTING PAIR BONDS WITH THEIR MATES.

A COUPLE MAY LIVE TOGETHER FOR A SEASON, OR EVEN A LIFETIME, WITH BOTH PARENTS CARING FOR THE EGGS, AND FEEDING THEIR GROWING CHICKS.

BUT GENETIC TESTING HAS PUT THE LIE TO THE MYTH OF MONOGAMY.

OUR STEREOTYPE OF A DEVOTED COUPLE BUILDING A LOVE NEST AND STAYING FAITHFUL FOREVER HAS GONE OUT THE WINDOW.

SCIENTISTS NOW KNOW THAT EVEN AFTER FEMALES CHOOSE A MATE TO LIVE WITH, THEY KEEP ON LOOKING FOR SOMETHING BETTER.

THERE'S A LOT OF CHEATING GOING ON, AND MORE IS MORE.

MORE COPULATION WITH ADDITIONAL MALES MEANS THAT YOU HAVE THE POSSIBILITY OF SIRING YOUR OFFSPRING WITH THE MATE THAT HAS THE BEST GENES.

Abraham: NOT ONLY BIRDS, BUT MANY OTHER ANIMALS ALSO SLEEP AROUND -- A LOT.

OF THE 4,000 OR SO SPECIES OF MAMMALS, VERY FEW FORM LASTING PAIR BONDS.

AND EVEN THOSE THAT DO HAVE CHEATING HEARTS.

RED FOXES WERE BELIEVED TO BE MONOGAMOUS, UNTIL DNA FINGERPRINTING PROVED IT WASN'T TRUE.

IN ONE STUDY, 80% OF RED FOX CUBS WERE THE PRODUCT OF EXTRACURRICULAR ACTIVITY BY FEMALES.

PROMISCUITY CAN BE A SMART MATING CHOICE.

Man: GENETIC VARIABILITY IS GOOD.

SO THAT GIVES THAT FEMALE AN ADVANTAGE IN EVOLUTION.

Abraham: BY CHEATING ON HER MATE, A FEMALE GETS A SECOND CHANCE TO FIND THE BEST FATHER FOR HER YOUNG -- ONE WITH GOOD GENES.

WHEN YOU'RE JUST STUCK WITH MR. NICE-GUY THAT YOU MET ON YOUR FIRST DATE, AND THAT'S YOUR LOT IN LIFE, THEN YOU MIGHT BE A BIT DISAPPOINTED.

IF YOU CAN ACTUALLY SHOP AROUND A BIT MORE, THEN YOU'VE GOT A BETTER CHANCE OF FINDING SOME REALLY GOOD GENES.

THESE FEMALES KEEP AN EYE OUT.

Woman: PROMISCUITY WORKS BECAUSE YOU CAN HEDGE YOUR BETS.

A FEMALE CAN AT LEAST LOOK LIKE SHE'S CHOOSING A MALE WITH ALL THOSE GOOD, "RAISE-MY-KIDS" KIND OF QUALITIES, WHILE BEARING A CHILD THAT HAS ALL THOSE SEXY, GET-THE-GIRL KIND OF QUALITIES AS A FATHER.

SHE HAS HER NICE DAD RAISE HER SEXY PLAYBOY SON.

Abraham: NOW THAT SOME TIME HAS PASSED, REBECCA'S EXPERIMENT SHOULD BE SHOWING RESULTS.

SHE CAN FIND OUT IF THOSE MAKEOVERS MADE A DIFFERENCE.

Safran: HERE IS A NEST WITH SIX NESTLINGS.

CAN YOU SEE THEIR LITTLE MOUTHS?

THEY'RE BEGGING FOR FOOD.

THIS IS THE NEST OF A MALE WHOSE FEATHER COLOR WAS DARKENED AT THE START OF THE SEASON.

Abraham: SO IT TURNS OUT THE FEMALE BARN SWALLOWS WERE COMPLETELY TAKEN IN.

THOSE MADE-OVER MALES WERE TRANSFORMED FROM LONELY GUYS INTO MACHO MEN... AND PROUD PAPAS.

THEY PRODUCED MORE YOUNG, AND DNA TESTS SHOW THAT THEY ACTUALLY FATHERED THOSE YOUNG.

THEIR MATES REMAINED MORE FAITHFUL.

Safran: SO, THIS TELLS US THAT FEATHER COLOR IS DEFINITELY A SIGNAL THAT INDIVIDUALS IN THIS SPECIES ARE PAYING ATTENTION TO.

Abraham: FOR MALE BARN SWALLOWS, DARK FEATHERS ARE THE KEY TO WINNING THE MATING GAME.

Safran: THE CURRENCY OF EVOLUTION, OF COURSE, IS THE NUMBER OF OFFSPRING YOU SUCCESSFULLY RAISE.

THUS, THE NEXT GENERATION IS CARRYING YOUR GENES.

Abraham: BUT THE MAKEOVER YIELDED ANOTHER RESULT... AND EVEN REBECCA WAS SURPRISED.

DARKENING THE MALES' FEATHERS DIDN'T JUST IMPROVE THEIR LOOKS, IT ACTUALLY CHANGED THEIR BODY CHEMISTRY.

BLOOD TESTS SHOW THAT THESE GUYS DEVELOPED HIGHER LEVELS OF TESTOSTERONE...

SOMETIMES EVEN HIGHER THAN MALES THAT WERE NATURALLY DARK.

IT COULD BE BECAUSE THE OTHER MALES CHALLENGED THEM MORE -- TESTOSTERONE GOES UP WHEN MALES FIGHT.

IT ALSO GOES UP WHEN MALES HAVE MORE SEX.

SO MAYBE ALL THAT ATTENTION FROM THE LADIES GAVE THEM A BOOST.

WHATEVER THE REASON, IT'S PRETTY CLEAR THAT, AMONG BARN SWALLOWS, CLOTHES DO MAKE THE MAN.

MALES THAT ARE SMALL, DARK, AND HANDSOME GET THE GIRLS.

Safran: MY SENSE IS THAT FEMALES ARE REALLY RUNNING THE SHOW HERE.

FEMALE CHOICE OF MALES BASED ON THEIR FEATHER COLORATION IS REALLY DRIVING THE EVOLUTION OF THIS SIGNAL IN THE SYSTEM.

Abraham: TO FIND OUT WHAT FEMALE GELADAS LOOK FOR WHEN THEY CHOOSE A MALE, CHADDEN WILL NEED TO LEARN MORE ABOUT THE ANIMALS' LIVES AND THEIR LIFESTYLE.

THE GELADA TROOP SPENDS MUCH OF THE DAY GRAZING.

THESE ARE THE ONLY PRIMATES THAT LIVE ON A DIET OF GRASS.

THE DAILY ROUTINE ALSO INCLUDES PLENTY OF GROOMING.

LIKE EVERY FAMILY MALE, CHEWBACCA DEVOTES A LOT OF TIME AND ENERGY TO HIS FEMALES.

IT'S ONE WAY TO KEEP THEM HAPPY AND STAY ON THEIR GOOD SIDE.

ANOTHER WAY IS BY SPENDING QUALITY TIME WITH THE KIDS.

Hunter: NOTICE THE BABY GET ON HIS BACK -- THAT'S GREAT BEHAVIOR.

THIS IS THE REAL...

THE REAL FAMILY MAN.

ALL THE FEMALES WOULD HAVE SEEN THAT AND THEY LOVE THAT.

IT'S JUST ONE OF THE STRATEGIES THAT SOME OF THESE MALES WILL TRY, IS THE BABYSITTING THING, WHERE THEY'RE REALLY FUN WITH THE KIDS, LOOK AFTER THEM.

TOTAL SUCK-UP JOB, BUT IT WORKS.

OH, HE'S JUMPED OFF.

[ GELADAS SHRIEKING ] Abraham: JUST AS WITH OTHER PRIMATES, GELADAS HAVE RELATIVELY LONG CHILDHOODS.

AND PLAY IS AN IMPORTANT PART OF EVERY YOUNG GELADA'S DEVELOPMENT.

YOUNG MALES ACTUALLY CROSS FAMILY BOUNDARIES TO PLAY WITH OTHER MALES THEIR OWN AGE.

TOGETHER, THEY PRACTICE SKILLS THEY'LL NEED LATER ON IN LIFE, WHEN THEY'RE COMPETING FOR A HAREM.

IN A FEW YEARS, THEY'LL BEGIN TO SPEND MORE TIME WITH THE BACHELOR GROUPS.

AND FINALLY THEY'LL LEAVE THEIR FAMILIES ALTOGETHER, TO JOIN THE BACHELORS.

BETWEEN CARING FOR THE KIDS AND GROOMING HIS HAREM, A FAMILY MALE LIKE CHEWBACCA HAS A LOT OF RESPONSIBILITIES.

HIS FEMALES KEEP HIM ON A SHORT LEASH.

Hunter: YOU CAN SEE CHEWBACCA SITTING IN FRONT OF ME HERE IN THE SUN, WARMING UP.

HE'S GOT ALL HIS GIRLS AROUND HIM.

HE'LL NEVER STRAY TOO FAR FROM THEM, BECAUSE THEY MAKE THE DECISIONS.

OOH, THERE'S ANOTHER BIG MALE.

THAT'S THE CHALLENGERS.

Abraham: ELVIS AND THE BACHELORS ARE PROWLING AROUND, READY TO TAKE ADVANTAGE OF ANY MISSTEP.

Hunter: JUST SEEING THE BACHELORS COME UP AND EVERYONE KNOWS THAT IF THESE GUYS DON'T MEASURE UP, THAT THERE'S THIS BIG POOL OF ELIGIBLE BACHELORS OVER HERE THAT THE GIRLS CAN CHOOSE TO REPLACE THEM WITH, GETS THEM ALL ON EDGE.

Abraham: TODAY THE BACHELORS SEEM ESPECIALLY BRAZEN, ASSESSING THE FAMILY MALES' WEAKNESSES.

Hunter: THERE'S SO MUCH TENSION HERE THIS MORNING.

JUST THINGS BUBBLING EVERYWHERE.

EVERYBODY'S A BIT ANGSTY.

THIS IS THE EXACT KIND OF ENVIRONMENT THAT A TAKEOVER COULD HAPPEN.

YOU CAN SEE CHEWBACCA DOWN FRONT HERE WITH ALL HIS FEMALES, THE CHEST PATCH IS SO BRIGHT.

AND ELVIS AND THE BACHELORS ARE JUST OVER HERE... Abraham: ELVIS IS EAGER FOR A REMATCH.

HE'S BEEN TESTING CHEWBACCA'S DEFENSES AND SHOWING OFF FOR THE FEMALES.

THIS TIME HE'S GOTTEN TOO CLOSE.

Hunter: OOH, HE'LL PAY FOR THAT.

Abraham: IT'S AN INDISCRETION THAT CHEWBACCA CANNOT PERMIT.

ELVIS HAS BEATEN A HASTY RETREAT.

THE CHALLENGER IS OUT OF THE WAY, AND FOR NOW, OUT OF THE RUNNING FOR FEMALES.

PUMPED UP BY HIS VICTORY, CHEWBACCA'S CHEST PATCH IS BLAZING CRIMSON, AND HE CAN CLAIM A REWARD FOR HIS EFFORTS.

HE'S HELD ONTO HIS TITLE FOR ANOTHER DAY.

FOR A FAMILY MALE, THE STAKES ARE HIGH, AND THE PENALTIES CAN BE SEVERE.

Hunter: SEE THIS GUY, BRUCE, JUST TRYING TO STARE AT ME?

HE USED TO HAVE A BIG FAMILY OF HIS OWN, BUT FOR SOME REASON THE GIRLS DECIDED TO GET RID OF HIM.

I DON'T KNOW WHETHER IT'S BECAUSE HIS MANE WASN'T LONG ENOUGH, OR WHETHER HE JUST WASN'T PLAYING HIS PART AROUND THE HOUSE.

BUT THIS IS WHAT HAPPENS WHEN A MALE GETS REJECTED.

HE NEVER GETS ANOTHER CHANCE.

THESE GELADA GIRLS ARE HARSH.

Abraham: CHADDEN BELIEVES CHEWBACCA'S FEMALES KEEP HIM AROUND, IN PART, BECAUSE OF HIS STRENGTH AND COURAGE.

THOSE ARE TRAITS THEY WANT PASSED ON TO THEIR OWN SONS.

IF CHADDEN'S GOING TO FIGURE OUT WHAT ELSE THEY SEEK IN A MATE, HE'LL NEED TO LOOK FOR NEW CLUES.

AFTER A LONG DAY OF GRAZING AND GROOMING, THE GELADAS ABANDON THE FAMILIAR LEVEL GROUND OF THE PLATEAU.

LOOK AT CHEWBACCA COMING TO THE EDGE.

AND IT'S THE WHOLE FAMILY.

JUST GOING OVER TO THE EDGE.

Abraham: THEY PREFER TO SPEND THEIR NIGHTS ON SLEEPING LEDGES, PERCHED ON THE VERTICAL WALL.

IT'S PRECARIOUS, AND EXPOSED TO THE ELEMENTS, BUT SAFE FROM LEOPARDS AND HYENAS.

Hunter: THERE'S JUST NO FURTHER I CAN GO DOWN THESE CLIFFS.

ALL THESE GUYS ARE JUST EXPERT ROCK CLIMBERS AND THEY JUST ROCKET DOWN THESE SHEER SLEEPING CLIFFS.

BUT THIS IS AS FAR AS I CAN FOLLOW THEM.

Abraham: THE NEXT PHASE OF CHADDEN'S RESEARCH INTO MALE CHEST PATCHES WILL CALL FOR SOME NEW TECHNOLOGY.

CHADDEN'S TRYING OUT A NEW HIGH-TECH TOY... A THERMAL IMAGING DEVICE.

IT REGISTERS THE HEAT ESCAPING FROM THE MONKEYS' BODIES, RENDERING THEM AS GHOSTLY SILHOUETTES -- THEIR BARE FACES AND CHESTS RADIANT AGAINST THE CHILL OF THEIR SURROUNDINGS.

Hunter: IT'S BEDTIME.

THIS CLIFF IS PITCH BLACK, BUT THIS THERMAL CAMERA IS PICKING UP EACH LITTLE GELADA AS THEY MOVE DOWN -- SEE THESE LITTLE BALLS OF WARMTH ROLLING DOWN LIKE DROPLETS.

IT'S FANTASTIC.

Abraham: FOR CHADDEN, IT COULD ALSO PROVIDE AN IMPORTANT CLUE TO HOW FEMALES CHOOSE.

THE NEXT DAY, THE TROOP IS ON THE MOVE, ROAMING ACROSS THE MEADOWS.

CHADDEN IS USING HIS THERMAL CAMERA TO MEASURE THE HIDDEN COSTS OF HAVING A RED CHEST PATCH.

THIS IS SO COOL.

YOU CAN JUST SEE HEAT POURING OFF THESE GUYS.

Abraham: THE CAMERA REVEALS WHAT CHADDEN SUSPECTED -- THOSE BARE CHEST PATCHES RELEASE A LOT OF BODY HEAT.

Hunter: NEVER SEEN THIS BEFORE.

YOU CAN SEE THE LITTLE PATCHES WHERE HE'S LACKING THAT FUR.

AND JUST THAT RED IS THE HEAT JUST COMING OFF HIS BODY.

Abraham: THE HEAT LOSS AND COLOR CHANGES ARE RELATED TO BLOOD FLOW.

THE REDDER THE PATCH, THE MORE BLOOD IS FLOWING JUST BENEATH THE SKIN, AND THE MORE HEAT IS ESCAPING.

Hunter: THERE IT IS.

HIS CHEST PATCH IS JUST LEAKING HEAT.

I JUST WONDER WHETHER THIS IS A WAY OF SHOWING OFF TO THE FEMALES.

THESE GUYS HAVE TO WITHSTAND SUBZERO CONDITIONS EVERY SINGLE NIGHT, AND BY SHOWING OFF, "LOOK, I CAN HANDLE THOSE CONDITIONS WITH THIS BARE CHEST PATCH," THAT'S PRETTY TOUGH.

Abraham: SCIENTISTS CALL THIS THE HANDICAP PRINCIPLE -- IF A MALE CAN AFFORD TO LOSE THAT MUCH HEAT, HE MUST HAVE ENERGY TO SPARE.

IT'S A TELLTALE SIGN OF FITNESS, LETTING A FEMALE KNOW HE'S A GOOD CHOICE AS A MATE.

AND IT CAN'T BE FAKED.

THIS KIND OF "HONEST SIGNAL" IS SEEN IN MANY SPECIES.

FLAMBOYANT DISPLAYS AND ORNAMENTS DEMONSTRATE THAT A MALE IS HEALTHY -- FREE OF DISEASE OR PARASITES.

Hunter: WHEN FEMALES ARE LOOKING AT MALES, THEY REALLY WANT AN HONEST ASSESSMENT OF HOW FIT THAT MALE IS.

AND SOMETIMES THAT MEANS MALES HAVE TO GO TO VERY DANGEROUS LENGTHS IN THEIR SHOWING OFF TO SHOW HOW TOUGH THEY ARE.

[ PEACOCK CALLING ] Abraham: THE MAGNIFICENT TAIL FEATHERS OF THE PEACOCK ARE A SURE SIGN THAT HE'S HEALTHY.

AND PEAHENS FIND THAT ATTRACTIVE.

HE MUST BE IN GOOD SHAPE TO GROW THAT PLUMAGE AND CART IT AROUND ALL DAY.

BY CREATING SUCH A SPECTACLE, THE PEACOCK ENSURES THAT HIS GENES WILL SURVIVE INTO THE NEXT GENERATION.

UNFORTUNATELY, HIS FEATHERS ARE A BIG TARGET FOR PREDATORS.

THAT MAKES IT LESS LIKELY THAT HE'LL SURVIVE INTO THE NEXT AFTERNOON.

FOR PEACOCKS OR GELADAS, SEXUAL ADAPTATIONS CAN EXACT A HIGH PRICE.

THEY SHOW HOW GREAT THE STAKES MUST BE FOR SEXUAL SUCCESS.

CHADDEN MAY HAVE FOUND ONE ANSWER TO THE MYSTERY OF THE RED CHEST PATCH.

BUT HE BELIEVES THE COLORS ARE SENDING ANOTHER SIGNAL.

THERE'S ONE MORE TEST HE'D LIKE TO TRY.

IT WILL PUT HIM IN MORE INTIMATE CONTACT WITH THE GELADAS THAN HE EVER ANTICIPATED.

ON THE SAGE GROUSE LEK IN WYOMING, THE BIRDS WILL ARRIVE AROUND DAWN.

SO GAIL AND HER TEAM HAVE TO GET THERE A LOT EARLIER THAN THAT.

Man: IS THAT EVERYTHING?

Woman: YEP.

ALL RIGHT, COOL.

Abraham: IT'S TIME TO HUNKER DOWN AND WAIT FOR THE ACTION TO BEGIN.

MALES SHOW UP FIRST TO STAKE OUT THEIR TERRITORIES, AND PATROL THEIR BORDERS.

ONE MALE GOT TOO FAR INTO ANOTHER MALE'S TERRITORY.

Abraham: FIGHTS CAN BREAK OUT ANYTIME BETWEEN NEIGHBORING MALES.

Patricelli: THEY'LL STAND OFF NEXT TO EACH OTHER, AND SHAKE THEIR TAIL FEATHERS, AND THEN IT'LL OFTEN ESCALATE TO SMACKING EACH OTHER OVER THE HEAD WITH THEIR WINGS.

Abraham: THEY'LL KEEP ON FIGHTING UNTIL ONE OF THEM BACKS DOWN.

THE WINNER HAS SECURED SOME PRIME REAL ESTATE.

THE STAGE IS SET FOR COURTSHIP.

IT'S TIME TO DEPLOY THE FEMBOT.

IT'S EXCITING.

Abraham: ALL THAT RESEARCH AND DEVELOPMENT GETS PUT TO THE TEST ON A REAL LEK, WITH REAL MALES.

Patricelli: OUR TARGET MALE IS STANDING OUT THERE AT THE END OF THE TRAIN TRACKS RIGHT NOW, WAITING FOR US.

SO LET'S SEE IF SHE'S GOING TO WORK.

ALL RIGHT, EVERYONE, HERE COMES THE ROBOT.

Abraham: AT LEAST ONE OF THE MALES LOOKS INTERESTED...

BUT IS HE REALLY INTERESTED IN THE FEMBOT?

Patricelli: LOOKS LIKE A REAL FEMALE IS APPROACHING.

MAKES IT A LITTLE HARD FOR US TO FIGURE OUT WHO'S BEING COURTED, IF IT'S THE ROBOT OR THE REAL FEMALE.

Abraham: SINCE THERE'S NO WAY FOR GAIL TO KNOW WHO THE MALE IS TARGETING WITH HIS CALLS, SHE WON'T BE ABLE TO DECIPHER HER RECORDINGS.

SO SHE DECIDES ON A TACTICAL RETREAT.

Patricelli: ALL RIGHT, NOW WE'RE GOING TO BRING HER BACK TOWARDS US.

OOP, THERE SHE GOES.

OH, NO!

OH, I GUESS I SHOULDN'T HAVE DONE THAT.

Abraham: GAIL DOESN'T WANT TO RETRIEVE HER FEMBOT UNTIL THE BIRDS HAVE LEFT THE LEK.

Patricelli: NOW WHAT DO WE DO?

Abraham: SHE HOPES NONE OF THE MALES TRIES TO TAKE ADVANTAGE OF THE SITUATION.

NO ONE'S MAKING A MOVE ON HER SO FAR, SO WE DON'T NEED A RESCUE.

Abraham: BY 9:00 A.M., THE LEK EMPTIES OUT, AND COURTING IS OVER FOR THE DAY.

Patricelli: SEE IF SHE'S GOING TO WORK AGAIN.

Abraham: THEY'LL HAVE TO WAIT UNTIL TOMORROW TO TRY ANOTHER RUN.

AND GAIL HOPES THAT NEXT TIME, THEY CAN GET THE RECORDINGS SHE NEEDS.

FOR MALES OF ANY SPECIES, COMMUNICATING THEIR MESSAGE IS ESSENTIAL IF THEY WANT A FEMALE TO CHOOSE THEM.

THE SOUTHERN ELEPHANT SEAL USES SOUND AS A SEXUAL SIGNAL.

[ BULL BELLOWING ] A BULL'S BELLOWING IS AMPLIFIED BY HIS BULBOUS NOSE.

THE SOUND NOT ONLY HELPS HIM INTIMIDATE RIVALS, IT ALSO LETS FEMALES KNOW HE'S AROUND, AND HE'S A FIT CHOICE FOR A MATE.

IT PAYS TO ADVERTISE.

BUT THERE ARE OTHER KINDS OF SEXUAL SIGNALS THAT WE CAN'T DETECT.

SCENT IS ONE OF THE MOST EFFECTIVE WAYS TO SIGNAL THE OPPOSITE SEX.

OLFACTORY MESSAGES MAY BE CONTAINED IN URINE, OR RELEASED BY SPECIAL GLANDS.

THEY'RE MEANT TO ATTRACT ATTENTION -- BUT NOT EVERYONE'S ATTENTION.

THE IDEAL SIGNAL WOULD GO UNNOTICED BY OTHER SPECIES, ESPECIALLY PREDATORS.

Woman: WITH ANY COMMUNICATIVE SIGNAL, YOU WANT TO MAKE IT AS CLEAR AS POSSIBLE TO YOUR INTENDED RECEIVERS, AND AS DIFFICULT TO HEAR AS POSSIBLE TO YOUR UNINTENDED RECEIVERS.

IT'S LIKE MILITARY INTELLIGENCE.

WE ONLY WANT ONE SET OF PEOPLE TO HEAR IT.

WE DON'T WANT OTHERS TO HEAR IT.

Abraham: MANY INSECTS SOLVE THAT PROBLEM WITH CHEMICAL SIGNALS CALLED PHEROMONES.

THEY'RE ESPECIALLY USEFUL FOR LOCATING A MATE.

LOTS OF ANIMALS HAVE VERY ELABORATE SENSORY STRUCTURES.

LIKE MOTHS HAVE THESE ELABORATE ANTENNAE FOR SENSING LITERALLY ONE MOLECULE OF A PHEROMONE.

AT NIGHT, A MOTH REALLY ONLY HAS THIS MOLECULE THAT'S WAFTING THROUGH THE SKY.

Abraham: THAT MOLECULE LOCKS ONTO A SPECIAL RECEPTOR ON THE MALE'S ANTENNA, HELPING HIM FIND A FEMALE FROM MILES AWAY.

SOME FEMALE MOTHS USE THEIR PHEROMONES AS A WAY OF TESTING A MATE'S FITNESS.

THE FEMALE EMITS HER PHEROMONE AT SUCH A LOW LEVEL THAT IT'S NOTICED ONLY BY MALES WITH THE VERY BEST CHEMICAL RECEPTORS.

THERE'S A LOT OF SEXUAL SIGNALING GOING ON BEYOND THE LIMITS OF OUR SENSES.

EVEN VISUAL SIGNALS MAY BE INVISIBLE TO US, WITHOUT HIGH-TECH TOOLS.

THE WINGS OF AFRICAN SATYRID BUTTERFLIES ARE DECORATED WITH CONSTELLATIONS OF EYESPOTS.

THEY'RE ARRANGED IN PATTERNS UNIQUE TO EACH SPECIES.

YALE BIOLOGIST ANTONIA MONTEIRO STUDIES THE EVOLUTION OF THESE PATTERNS.

Monteiro: THEY SEEM TO USE THESE EYESPOTS TO COMMUNICATE WITH EACH OTHER DURING COURTSHIP.

THEY SEEM TO BE VERY IMPORTANT IN TERMS OF FEMALE CHOICE.

Abraham: SCIENTISTS HAVE ASSUMED THAT FEMALES ARE DRAWN TO MALES WHOSE EYESPOTS HAVE THE LARGEST WHITE PUPILS.

ANTONIA HAS COME UP WITH A DIFFERENT THEORY.

SHE HAS OBSERVED THAT FEMALES RESPOND ONLY TO THE EYESPOTS ON THE UPPER SIDES OF THE MALE'S WINGS.

THE EYESPOTS REFLECT ULTRAVIOLET LIGHT.

LIGHT IN THE ULTRAVIOLET SPECTRUM IS INVISIBLE TO HUMANS, BUT MANY OTHER ANIMALS ARE ABLE TO SEE IT... AND USE IT FOR SEXUAL DISPLAYS.

WITH A SPECIAL UV CAMERA, WE CAN GET AN IDEA OF WHAT THE WORLD LOOKS LIKE TO A SATYRID BUTTERFLY.

THOSE REFLECTIVE EYESPOTS CATCH A FEMALE'S EYE, ESPECIALLY WHEN THE MALE FLUTTERS HIS WINGS DURING COURTSHIP.

Monteiro: LOOK, THE MALE TRYING TO COURT WITH THE FEMALE.

Abraham: ANTONIA WONDERS IF FEMALES USE THIS ULTRAVIOLET FLASHING AS A WAY OF SELECTING THEIR MATES.

TO TEST THE IDEA, SHE'LL BLOCK THE REFLECTIONS WITH A LITTLE PAINT.

CAREFULLY SEPARATING A MALE'S WINGS, SHE DABS A TRANSPARENT SOLUTION ON HIS UPPER EYESPOTS.

IT'S A PLANT EXTRACT THAT WORKS LIKE SUNBLOCK, STOPPING THE PASSAGE OF UV RAYS.

IN NORMAL LIGHT, THE EYESPOTS STAY WHITE.

BUT UNDER ULTRAVIOLET LIGHT THEY DISAPPEAR.

ANTONIA PLACES TWO MALES IN THE PRESENCE OF A SINGLE FEMALE.

ONE MALE HAS HIS EYESPOTS PAINTED OVER, THE OTHER RETAINS HIS NORMAL GLEAM.

JAMMING HIS SEXUAL SIGNALS BY BLOCKING THE REFLECTIONS EFFECTIVELY PUTS AN END TO A MALE'S CHANCES OF MATING.

THE FEMALE JUST ISN'T INTERESTED.

AS FOR THE MALE WHOSE EYESPOTS CONTINUE TO SHINE, HIS WINGS GIVE HIM THE WINNING EDGE.

SO, AMONG SATYRID BUTTERFLIES, IT'S NOT THE SIZE OF THE SPOTS THAT MATTERS, IT'S THEIR SPARKLE.

THAT'S THE SIGNAL THE FEMALES ARE LOOKING FOR, EVEN IF IT'S INVISIBLE TO US.

THE SIGNALS OF SAGE GROUSE SEEM A LOT MORE OBVIOUS.

BUT GAIL IS CERTAIN THAT THERE ARE SUBTLE NUANCES THAT HAVE GONE UNNOTICED.

WITH NO FEMALES IN THE TARGET MALE'S TERRITORY, THIS IS A PERFECT TIME TO SEND THE FEMBOT BACK OUT ON THE TRACK.

SURE ENOUGH, THE BOYS ARE SOON COMPETING FOR HER ATTENTION.

ONE MALE IS PUTTING ON A REALLY BIG SHOW.

THERE CAN BE NO DOUBT ABOUT HIS INTENTIONS -- HE'S DISPLAYING FOR THE FEMBOT.

GAIL'S EXPERIMENT IS WORKING.

AS HIS CALLS ARE RECORDED BY THE WEB OF MICROPHONES, CRUCIAL DATA IS BEING COLLECTED, ABOUT THEIR VOLUME AND QUALITY, HOW HE AIMS HIS CALLS TOWARD THE FEMBOT, AND HOW HE RESPONDS TO HER CUES.

WHEN SHE APPROACHES, HE INCREASES THE RATE OF HIS CALLS, TURNING ON ALL THE ARTS OF SAGE GROUSE PERSUASION.

Patricelli: TO BE ABLE TO ORIENT IT APPROPRIATELY TO FEMALES, TO INTERACT APPROPRIATELY WITH FEMALES, POSITION HIMSELF... ALL OF THESE KINDS OF SOCIAL SKILLS THAT ARE IMPORTANT IN SEXUAL SELECTION.

Abraham: IN A NEIGHBORING TERRITORY, ANOTHER MALE IS MAKING HEADWAY, WITH A REAL FEMALE.

Patricelli: OH, THERE THEY GO.

Abraham: ONCE SHE CHOOSES HIM, THEIR MATING IS OVER IN THE BLINK OF AN EYE.

THE FEMBOT COULD HELP GAIL FIGURE OUT HOW SHE MADE HER CHOICE.

[ REPLAYING THUMPING ] THE TEAM PORES OVER THE AUDIO DATA FOR CLUES TO WHAT'S GOING ON OUT ON THE LEK, AND WHAT THESE FEMALES REALLY WANT.

IN TWO SEASONS ON THE LEK, GAIL HAS RECORDED MORE THAN 4,000 HOURS OF SAGE GROUSE CALLS -- MUCH OF IT CORRELATED WITH VIDEO OF DISPLAYS AND MATING.

IT'S BECOME CLEAR THAT FEMALES AREN'T PERSUADED BY A MALE'S FLASHY FEATHERS AND SMOOTH MOVES.

THE VISUAL DISPLAY DOESN'T SEEM TO BE CRITICAL EVEN THOUGH IT'S SO SPECTACULAR.

THE MOST IMPORTANT THING SEEMS TO BE THE AMOUNT OF TIME THEY SPEND ON THE LEK AND THE RATE OF THEIR DISPLAY, WHICH ARE BOTH QUANTITY THINGS.

IT'S JUST SORT OF "SHOW UP AND WORK HARD."

Abraham: PERSISTENCE PAYS OFF.

RAPID CALLS OVER A LONG PERIOD OF TIME SHOW THAT A MALE HAS ENERGY TO SPARE, THAT HIS GOOD GENES MAKE HIM A GOOD CHOICE.

BUT FEMALES ARE TOUGH CRITICS.

HE HAS TO SING JUST THE RIGHT TUNE.

Patricelli: AS FAR AS THE QUALITY OF THE DISPLAY, THERE'S A PARTICULAR ASPECT OF THE VOCALIZATION THAT SEEMS TO BE IMPORTANT.

AND IT'S VERY DIFFICULT TO HEAR, FOR US, BUT IT SEEMS TO BE IMPORTANT TO THE FEMALES.

THEY MAKE A LITTLE POP, AND THEN A WHISTLE, AND THEN A POP.

IT SOUNDS "BOM-BOMP-BOM."

[ THUMPING, CALLING ] Patricelli: THE LOUDNESS OF THAT WHISTLE SEEMS TO BE IMPORTANT, AND THE TIME LAPSE BETWEEN THE TWO POPS.

[ CALLING ] AND THAT'S WHAT FEMALES CHOOSE THEIR MALE ON.

AND THE AMAZING THING IS HOW HARD IT IS FOR US TO HEAR THE KINDS OF THINGS THAT ARE IMPORTANT TO FEMALES.

Abraham: IT MAY BE SUBTLE, BUT FOR A FEMALE SAGE GROUSE, IT'S ALL IN HIS CALL.

THE FEMBOT IS HELPING TO SOLVE ONE OF THE MYSTERIES OF SEXUAL SELECTION.

BUT THERE'S A LOT MORE WORK TO BE DONE.

GAIL AND THE FEMBOT WILL BE BACK ON THE LEK NEXT YEAR.

TO COMPLETE HIS PICTURE OF GELADA BEHAVIOR, CHADDEN HUNTER IS GOING WHERE HE'S NEVER GONE BEFORE...

HE'S CHECKING OUT THE GELADAS' SLEEPING LEDGES.

Hunter: OH, WOW.

[ GELADAS SHRIEKING ] WHAT A PLACE FOR A MONKEY TO SLEEP.

THIS IS CRAZY.

[ CHATTERING ] Abraham: SCAMPERING ACROSS THE CLIFF-FACE, THE GELADAS SEEM TO DEFY GRAVITY.

THEY MAKE IT LOOK SO EASY, IN SPITE OF THE RISKS OF A 3,000-FOOT DROP TO THE ROCKY FLOOR.

Hunter: THESE GUYS ARE MASTER ROCK CLIMBERS.

ON THESE SHEER, SHEER CLIFFS.

THEY'RE JUST OVER THERE ON THE LEDGE BESIDE ME.

IT'S THE FIRST TIME I'VE EVER HAD A VIEW LIKE THIS OF THEM.

[ CHATTERING ] SUCH AMAZING SKILL, IT MAKES YOU... MAKES YOU FEEL SO CLUMSY, AS A HUMAN.

[ CHATTERING ] I'M BEGINNING TO UNDERSTAND WHY THESE MONKEYS DO IT -- THE VIEW FROM UP HERE IS JUST ABSOLUTELY SPECTACULAR.

Abraham: FOR HIS EFFORTS, CHADDEN HAS GOTTEN A NEW PERSPECTIVE ON THE GELADAS' SLEEPING HABITS.

Hunter, panting: OH, MAN...

I WASN'T BORN TO BE A GELADA.

Abraham: AS THE SUN CLIMBS HIGHER IN THE SKY, THE GELADAS CLIMB BACK UP TO THE PLATEAU.

AND CHADDEN CONTINUES PROBING THE LINKS BETWEEN MALE CHEST PATCHES AND FEMALE CHOICE.

HE KNOWS FAMILY MALES HAVE TO BE STRONGER, TOUGHER, AND MORE VIRILE.

IT STANDS TO REASON THEIR HORMONE LEVELS MAY BE DIFFERENT, TOO.

A REDDER CHEST COULD LET FEMALES KNOW THAT A GUY IS WORTH CHOOSING BECAUSE HE HAS THE RIGHT CHEMICAL STUFF.

TRYING TO TAKE BLOOD SAMPLES FROM A FIERCE MALE GELADA WITH BIG TEETH AND A BAD ATTITUDE, IS JUST NOT PRACTICAL.

THERE'S ONLY ONE GOOD WAY TO FIND OUT WHAT THESE MEN ARE MADE OF.

Hunter: THIS IS THE FUN PART.

I'M TRYING TO GET IN HERE, WAITING FOR CHEWBACCA TO GO TO THE BATHROOM THIS MORNING.

Abraham: CHADDEN IS STALKING HIS TARGET MALES -- THE ONES HE'S ALREADY PHOTOGRAPHED -- SO HE CAN COLLECT THEIR DUNG, AND CHECK THEIR HORMONES.

Hunter: HE EATS ABOUT TWO POUNDS OF GRASS PER DAY.

HE'S GOT TO GO TO THE BATHROOM AT SOME STAGE.

I MEAN, THAT IS A LOT OF FIBER.

THIS IS A REALLY BORING PART OF THE JOB, WAITING ALL MORNING FOR A MONKEY TO POOP.

Abraham: HE'LL HAVE TO LET NATURE TAKE ITS COURSE.

BINGO.

Abraham: NOW IT'S TIME TO SCOOP THE POOP.

CHADDEN IS GETTING MORE INTIMATE WITH THESE MONKEYS THAN HE'D LIKE... OH!

BUT IT'S ALL FOR SCIENCE.

Hunter: SO, THIS IS WHERE ALL THE MONKEY POOP ENDS UP.

I'VE GOT THIS LITTLE SOUVENIR FROM EACH ONE OF MY FRIENDS.

Abraham: CHEMICAL ANALYSIS CONFIRMS HIS THEORY -- GELADAS WITH REDDER CHESTS REALLY DO HAVE HIGHER LEVELS OF HORMONES.

THE CHEST PATCHES COULD SERVE AS A WINDOW INTO THEIR GENETIC MAKEUP, TO HELP A FEMALE PICK OUT THE FITTEST MALES -- AND PRODUCE EQUALLY FIT OFFSPRING.

BUT CHADDEN BELIEVES THE FEMALES AREN'T JUST LOOKING FOR HORMONES, THEY WANT THE WHOLE PACKAGE.

Hunter: SO WHAT I'VE LEARNT IS THAT THESE GELADA FEMALES ARE LOOKING FOR A MALE THAT'S SENSITIVE, WHO'LL HELP GROOM THEM, WHO'S HANDSOME -- HAS THAT BRIGHT RED CHEST PATCH.

BUT MOST OF ALL, HE'S GOT TO BE TOUGH, AND HELP DEFEND THE FAMILY.

Abraham: CHEWBACCA'S REIGN WON'T LAST FOREVER.

THE DEMANDS OF MATES AND YOUNG, AND HIS BATTLES WITH BACHELORS, WILL TAKE THEIR TOLL.

BUT BEFORE A CHALLENGER CAN TAKE OVER A FAMILY, HE'LL HAVE TO CONVINCE THE FEMALES THAT HE'S WORTHY.

BECAUSE ULTIMATELY, THE CHOICE IS UP TO THEM.

SCIENTISTS HAVE SHOWN HOW FEMALES CAN SHAPE THE FUTURE OF THEIR SPECIES BY CHOOSING THE BEST MATES.

YET THERE'S ANOTHER SIDE TO THIS EQUATION... Bostwick: THERE'S A HUGE AMOUNT OF COMPETITION AMONG MALES TO BE THAT ONE GUY.

Sinervo: THE SHOWIEST MALES, OR THE STRONGEST OR THE SNEAKIEST, GET THE MOST MATES.

Patricelli: FIGHTS CAN BE VERY VICIOUS.

IT'S EXTREMELY BLOODY.

Abraham: THERE SEEMS TO BE NO END TO WHAT MALES WILL DO.

IT'S KIND OF LIKE HE'S TRYING TO HYPNOTIZE HER.

[ BUZZING ] Woman: LOOK AT THAT... ♪♪♪ THIS IS A REAL BATTLE OF THE SEXES.